A Binary Ionogel Electrolyte for the Realization of an All Solid‐State Electrical Double‐Layer Capacitor Performing at Low Temperature

Over the last years, solid‐state electrolytes made of an ionic liquid (IL) confined in a solid (inorganic or polymer) matrix, also known as ionogels, have been proposed to solve the leakage problems occurring at high temperatures in classical electrical double‐layer capacitors (EDLCs) with an organic electrolyte, and thereof improve the safety. However, making ionogel‐based EDLCs perform with reasonable power at low temperature is still a major challenge due to the high melting point of the confined IL. To overcome these limitations, the present contribution discloses ionogel films prepared in a totally oxygen/moisture‐free atmosphere by encapsulating 70 wt % of an equimolar mixture of 1‐ethyl‐3‐methylimidazolium bis(fluorosulfonyl)imide and 1‐ethyl‐3‐methylimidazolium tetrafluoroborate – [EMIm][BF 4 ] 0.5 [FSI] 0.5 – into a poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVdF‐HFP) network. The further called “binary ionogel” films demonstrated a high flexibility and a good ionic conductivity of 5.8 mS cm −1 at 20 °C. Contrary to the ionogels prepared from either [EMIm][FSI] or [EMIm][BF 4 ], displaying melting at T m =−16 °C and −7 °C, respectively, the crystallization of confined [EMIm][BF 4 ] 0.5 [FSI] 0.5 is quenched in the binary ionogel, which shows only a glass transition at −101 °C. This quenching enables an increased ionicity and ionic diffusion at the interface with the PVdF host network, leading the binary ionogel membrane to display higher ionic conductivity below −20 °C than the parent binary [EMIm][BF 4 ] 0.5 [FSI] 0.5 liquid. Laminate EDLCs were built with a 100 μm thick binary ionogel separator and electrodes made from a hierarchical micro‐/mesoporous MgO‐templated carbon containing a reasonable proportion of mesopores to enhance the mass transport of ions, especially at low temperature where the ionic diffusion noticeably decreases. The EDLCs operated up to 3.0 V with ideal EDL characteristics from −40 °C to room temperature. Their output specific energy under a discharge power of 1 kW kg −1 is ca. 4 times larger than with a cell implementing the same carbon electrodes together with the binary [EMIm][BF 4 ] 0.5 [FSI] 0.5 liquid. Hence, this binary ionogel electrolyte concept paves the road for developing safe and flexible solid‐state energy storage devices operating at subambient temperatures in extreme environments.